Incandescent lamps exhibiting the so-called halogen cycle are well known in the art. Such lamps operate on a tungsten-halogen cycle which is a regenerative, continuous process in which tungsten-halide is produced when the halide combines chemically with particles of tungsten evaporating from an incandescent tungsten filament. Subsequent thermal decomposition of this compound replaces the tungsten particles on the filament. At high temperatures, this process keeps the tungsten particles from depositing on the lamp envelope and producing a black coating which reduces light output.
The halide additive often used is hydrogen bromide (HBr) in its gaseous state. In the vicinity of the lighted filaments the HBr disassociates and forms free bromine (Br) which migrates to the vicinity of the envelope wall where it combines with tungsten which has evaporated from the filament. The bromide taking part in the regenerative cycle is tungsten bromide. If any part of the envelope wall is at or below 473.degree. K. (200.degree. C.), the tungsten bromide will chemically decompose in this region, yielding free bromide and metallic tungsten, the latter of which subsequently condenses on the envelope wall. If, on the other hand, the entire envelope wall is above 473.degree. K. (200.degree. C.), the tungsten bromides will be chemically stable, and will migrate back to the filament. Thus at temperatures below 473.degree. K. (200.degree. C.), the tungsten halogen cycle fails due to the lack of stability of tungsten bromide.
The failure of the halogen cycle to operate efficiently at lower temperatures has caused significant problems in lower wattage lamps which normally have corresponding cooler envelope walls. Specifically, the envelope walls will go black due to metallic tungsten deposits, indicating a failure of the halogen cycle. This will lead to reduced light output and early failure. If this problem could be eliminated, then the advantages of a properly operating halogen cycle could be extended to lamp types previously thought to be operating at too low an envelope wall temperature.
It is believed, therefore, that a halogen lamp and fill gas mixture that reduces the lower operating temperature limit of the halogen cycle and greatly increases its efficiency would constitute a significant advancement in the art.